1. Field of the Invention
The invention pertains to the field of catalysts and to methods of making and using them. The catalysts are used in the production of linear polyesters through the esterification or transesterification and polycondensation of multivalent alcohols with multivalent carboxylic acids.
2. Summary of the Related Art
The production of polyesters generally takes place in a manner in which a diol is converted to the corresponding dicarboxylic acid ester with a dicarboxylic acid or a low weight dicarboxylic acid ester, e.g., dimethyl ester, the dicarboxylic acid ester undergoing single or multistage polycondensation with increasing temperatures under decreased pressure. Various catalysts are used for the esterification and/or the interchange of ester radicals and polycondensation depending on the selection of the diols and dicarboxylic acids.
Catalysis in the production of polyester (particularly of the polycondensation reaction to build the polymer chain of linear polyester according to the chemical principle of polyesterification and poly-interchange of ester radicals) takes place as a result of the addition of metals that are capable of forming transition complexes with functional groups, in particular with the hydroxyl end groups. Preferred metal ions are those with three or more free valences, such as Sb3+, Pb4+, Ti4+, Bi3+, As3+, Si4+, Sn4+, Al3+ and Ge4+; these are effective catalysts for polycondensation. In industrial practice, Sb3+ and Ge4+ have proven to be preferred for the polycondensation of polyethylene terephthalate (PET), whereby metal concentrations of 150 to 300 ppm Sb and 20 to 120 ppm Ge are effective, depending on the type of polycondensation reactor; Ti4xe2x88x92 in the form of, for example, its tetra alkoxide has proven useful as a catalyst in the production of polybutylene terephthalate (PBT) and polytrimethylene terephthalate (PTT) and their co-polyester due to its generally high reactivity and high hydrophilicity. For the production of PET there has been much experimentation regarding the use of Ti as a catalyst, primarily because of the possible environmentally damaging effects of Sb as a heavy metal.
Above all, the use of complex-forming ligands such as tartaric acid or citric acid according to DD-A-286 174 and WO 97/47675, or the formation of complex TiO2xe2x80x94SiO2 compounds according to U.S. Pat. No. 5,789,528, or the use of alkali modified titanium oxide according to EP-A-0 826 713, EP-A-0 827 518 and EP-A-0 799 268, or the use of mixed catalysts of Ti with other metals, as these are described in U.S. Pat. No. 5,340,907, DE-A-196 27 591 and EP-A-0 745 630, should, on the one hand, reduce the high sensitivity of the titanium alkoxides to hydrolysis and stop the associated inclination to precipitate, and on the other hand should reduce the formation of colored complex compounds that are brought about by the titanium. In many of these patent publications a concentration range of 5 to 100 ppm Ti, preferably 5 to 30 ppm, is indicated.
The following table (Table 1) lists several patent publications that disclose Ti in combination with various additions as catalysts, whereby the range of yield and the reprocessed, exemplified embodiments are compared.
The need for a Ti concentration of  greater than 20 ppm (as required in practice) is principally evident from the fact that in industrial scale polycondensation, part of the metal catalyst is converted by traces of water to catalytically inactive TiO2 precipitate. This precipitate leads to increased turbidity and/or shell-like deposits in the polyesters. Moreover, the part of the catalyst metal converted into TiO2 is unavailable for polycondensation. Aside from that, higher concentrations of titanium cause a yellow color in the polyester, despite inhibition and addition of high concentrations of dye, and result in a polyester that is less brilliant overall and thermally unstable.
It is also known that Sb in the presence of water forms antimony oxide hydrates or, in the presence of phosphoric acid, e.g., forms antimony phosphates. The reduction of Sb to catalytically inactive antimony metal results in a gray color in the polyester.
The present invention comprises a composition, a method for its production, and the use of the composition for the production of linear polyester having the lowest possible content of catalytically active metal compounds and that may be used, in particular, in the production of bottles, films and other foodstuff packaging, as well as for filaments, threads, and formed bodies. The composition comprises a fine grain, porous carrier into which a catalytically active metal compound has been adsorbed.
One embodiment of the invention is a composition of porous, fine grain particles made of a heterogeneous carrier substance containing catalytically active metal compounds adsorbed on the surface of the particles and on the surface within the pores of the particles. This composition affords protection of the catalytically active metal compound in the reaction medium and subsequent diffusion of the catalyst to the site of the reaction.
One advantage of the catalyst of the invention arises from the fact that the carrier substance comprises at least one substance selected from among activated charcoal, diatomaceous earth, zeolites, synthetic and natural silicates, silicone oxides, silicone oxide hydrates, fuller""s earth, cross linked polymers and the like, although the principle of action is not limited to the indicated substances. The carrier substance must be able to adsorb the catalytically active metal compounds used in the invention. The particles of the carrier substance of the invention have an average grain size (d50) of 0.05 to 2 xcexcm, preferably 0.1 to 1 xcexcm, and most preferably 0.2 to 0.5 xcexcm. In the reaction medium these particles do not agglomerate substantially.
The specific surface area of the particles that form the carrier substance, as determined according to the BET method, is xe2x89xa750 m2/g, preferably  greater than 500 m2/g.
Catalytically active metal compounds that can be used with the carrier substance of the invention are metal compounds of one or more of, for example, Ti, Sb, Ge, Sn and Al that supply the polyester reaction mixture with Ti4+, Sb3+, Ge4+, Sn4+, and Al4+ ions, respectively, for reaction with the OH or COOH end groups of the polyester reaction mixture. Suitable catalytically active metal compounds include salts, oxides, and organic compounds of the metals.
In one embodiment of the invention, the ratio by mass or weight of the carrier substance to the catalytically active metal compound is 1:50 to 50:2, preferably 1:2 to 20:1.
The composition according to the invention is produced by reacting the carrier substance and the catalytically active metal compound by suspending them in an anhydrous liquid, preferably homogeneously suspended in the multi-valent alcohol selected for the production of the polyester. It is advantageous to carry out the reaction at increased temperature, i.e., up to the boiling point of the anhydrous liquid (e.g., alcohol) used.
The purity of the composition can be increased after cooling by separating the anhydrous liquid with parts of catalytically active metal compounds that are not adsorbed in the pores of the carrier by mechanical means (e.g., centrifugation or filtration) after cooling the reaction suspension, and, if necessary, washing the residue. For simplicity, the produced composition can be stored as a suspension in the multivalent alcohol selected for the corresponding polyester process, in particular a diol.
The finest comminution of the carrier substance can expediently be made by grinding, e.g., in an impacting mill, or by milling the suspension containing the carrier substance, e.g., according to the bead-mill principle. In particular cases it may be necessary to mill the suspension once the reaction between the carrier substance and the metal compound has been terminated in order to maintain a grain distribution suited to the application.
During the production of linear polyesters using the catalyst formed according to the invention, the amount of catalyst required for the polycondensation and interchange of ester radicals is 0.1 to 100 parts of the catalyst""s metal content per million parts of PET.
During the production of linear polyester or copolyesters through the interchange of ester radicals of a dicarboxylic acid dialkyl ester or through esterification of a dicarboxylic acid with a diol and subsequent one- or multi-step polycondensation, the suspension containing the composition of the invention is added during the time period from before the beginning of the esterification to just until shortly before the end of the polycondensation.
In the transesterification process, the catalyst according to the invention is added after the blocking of the transesterification catalyst, since the catalytic activity of the metal compounds adsorbed to the carrier substance can be greatly limited by the phosphorous compounds that are usually used for blocking of the transesterification catalysts.
A further advantage of the method according to the invention is that the amount of time for adding a stabilizer, such as phosphorous acid, phosphoric acid, phosphonic acid, and carboxyphosphonic acid and its compounds (at a quantity of 1 to 50 ppm, preferably 1 to 10 ppm, relative to the polyester mass) and the time at which the composition according to the invention is added is as long as possible. Thus, for example, the composition of the invention is added to the monomer mixture in the esterification process at the start, and the phosphorus-containing stabilizer is added, at the earliest, after the supply of the total monomer mixture has been completed, corresponding to a degree of esterification of 60 to 98%. For a non-catalyzed esterification the phosphorous-containing stabilizer can be added at the beginning of the esterification and at the end of the esterification.
In the production of polyester, a heavy metal-free blue dye can be added to the reaction mixture in quantities up to 3 ppm. Dyes that may be added include organic blue or blue-red dyes, in particular dyes permissible for foodstuffs, such as Estofil blue SRB-L and Estofil red SGF-T, produced by the Clariant company.
The addition of polyfunctional alcohols such as tri- or tetrahydroxy alkane, polyfunctional carboxylic acids and phosphonic acid compounds, such as are described, e.g., in U.S. Pat. No. 5,744,572 and U.S. Pat. No. 5,756,033, can also be advantageous in concentrations up to 300 ppm relative to the polyester mass. The concentration of the catalyst can thus be reduced even further.
In the examples below, values were determined as follows.
The intrinsic viscosity (I.V.) was measured at 25xc2x0 C. on a solution of 500 mg polyester in 100 ml of a mixture of phenol and 1,2-dichlorbenzol (3:2 parts by weight).
The COOH end group concentration was determined by means of photometric titration of a solution of polyester in a mixture of o-cresol and chloroform (70:30 parts by weight) with 0.05 N ethanolic caustic potash solution with bromthymol blue.
The determination of diethylene glycol (DEG), isophthalic acid (IPA) and 1,4-cyclohexane dimethanol (CHDM) in polyester took place by means of gas chromatography following a previous methanolysis of 1 g polyester in 30 ml methanol with the addition of 50 mg/l zinc acetate in a bomb tube at 200xc2x0 C.
Free glycol (EG) was determined by means of conversion with periodic acid to formaldehyde, reduction of the excess periodic acid with potassium iodide and titration of the liberated iodine.
The saponification number was determined through saponification with potassium hydroxide in n propanol and potentiometric titration in dimethyl formamide. The degree of esterification (U) was calculated from the saponification number (Vz) and the acid number (Sz) of the reaction mixture according to U=(Vzxe2x88x92Sz) 100/Vz.
The measurement of turbidity in nephelometric turbidity units 2 (NTU) took place on a 10 mass % solution of polyester in phenol/1,2-dichlorobenzol (3:2 parts by weight) with a nephelometer from the Hach company (type XR, according to U.S. Pat. No. 4,198,161) in a cuvette with a diameter of 22.2 mm analogous to norm DIN 38404, part 2 as is used for water. The intensity of the scattered light is measured in comparison to a formazine standard solution minus the value of the solvent (approximately 0.3 NTU).
The measurement of the color values L and b took place according to the Hunter L.a.b color measuring system (similar to Cielab). The polyester chips are first crystallized in a drying cabinet at 135+/xe2x88x925xc2x0 C. for one hour. The color values were then determined. The color tone of the polyester sample is measured in a three area color measurement device with three photocells, each of which has a red, green and blue filter connected thereto (X, Y and Z values). The evaluation was carried out according to the formula of Hunter, whereby
L=10{square root over (Y)}, and
  b  =                    7.0        ·                  (                      Y            -                          0.8467              ⁢                              xe2x80x83                            ⁢              Z                                )                            Y              .  
The acetaldehyde was driven from the polyester by heating in a closed container, and the acetaldehyde in the gas space of the container was determined by gas chromatography by means of head space analysis (gas chromatograph with FID and head space injection system HS40, Perkin Elmer; carrier gas:nitrogen; column: 1.5 m stainless steel; filling: Poropak Q, 80-100 mesh; sample quantity: 2 g; heating temperature: 150xc2x0 C.; duration of heating: 90 min.).
Definitions
The term polyester refers to polymers comprising terephthalic acid or 2,6-naphthaline dicarboxylic acid and ethylene glycol, 1,3-propanediol, 1,4-butanediol and/or 1,4-clycohexane dimethanol and its copolymers with other dicarboxylic acids, such as, for example, isophthalic acid, terephthalic acid, 1,4-cyclohexane dicarboxylic acid, 1,6-naphthaline dicarboxylic acid, p-hydroxy benzoic acid, 4,4-bisphenyl dicarboxylic acid, adipinic acid, phthalic acid, alkane dicarboxylic acid and/or diols, such as diethylene glycol, 1,4-butandiol, 1,4-cyclohexane dimethanol and polyglycol with a molecular weight under 1000. The preferred polyester is polyethylene terephthalate that contains 0.5 to 5.0 mass % diethylene glycol and 0 to 5.0 mass % isophthalic acid, 2,6-naphthaline dicarboxylic acid, p-hydroxybenzoic acid and/or 1,4-cyclohexane dimethanol as a co-monomer. The catalysts according to the invention are also suitable for the polycondensation of hydroxycarbonic acids such as, e.g., lactic acid or glycolic acid.